The invention relates to compression coding of a video program, and more particularly to an adaptive method for encoding successive frames of the video program.
As the costs of high-resolution color computer displays and processing power come down, one of the emerging applications for microcomputers is video post production--displaying and editing video images using the display of the computer as the monitor during the editing process. In order to use a microcomputer in a video editing system, a video source, typically a video tape recorder, is read and stored in digital form on the disk of a computer. The video may be edited in digital form and written back to a video device. Video editing presents a large computational and storage demand, easily seen in the sheer data volume of a video program--30 frames per second, over 300,000 pixels per frame, and several bits per pixel. In order to reduce the data volume, the video image data can be compressed as they are read, e.g., from video tape, and stored on disk. The data are then decompressed when viewed during editing or playback.
Selecting a video data compression method is a tradeoff between quality and quantity. More aggressive compression methods will reduce the amount of compressed data, but may result in lower-quality decompressed images. Generally, recovering the quality of the decompressed image requires the use of less-aggressive compression. Data compression algorithms generally have one or more adjustable parameters that control this tradeoff between quality and quantity; these coefficients are called "quantization factors" or "Q-factors."
The amount of compressed data produced for a frame of a video program will vary frame-to-frame as the content of the frames varies. In a known prior video compression method, a single set of Q-factors was used to encode each clip of a video program to be edited. The result was that the easy-to-compress program material, material that could have retained adequate quality even at more aggressive compression levels, consumed more data than required to present a good picture. Other harder-to-compress program material, material that required less-aggressive compression to avoid compression artifacts, was recorded at compromised quality. Additionally, when such material was edited, the editor was constrained to edit together only material that had been recorded with the same Q-factors.